Anticyclones or high-pressure systems

Figure 14

Anticyclones or high-pressure systems are the opposite of low pressure systems – they are divergent (winds diverge outwards from their centres) and air descends from upper parts of the troposphere above their centres, usually resulting in adiabatic warming of the troposphere and the evaporation of clouds in  the mid-troposphere. Near the surface during winter-time and over cold ocean surfaces, however, shallow surface inversions may form, bringing cold, frosty or foggy weather conditions (McIlveen, 2010) – see Figure 15.

In terms of air pressure, surface values in the centre of “highs” are typically 3-5% above the global mean average (i.e. ~1020 to ~1040hPa). Furthermore, winds surrounding a high pressure are much lighter due to a gentler pressure gradient (the isobars are widely space on a synoptic weather chart).

However, there are important differences between low and high-pressure systems, principally in terms of mobility/duration of a “high” and average windspeed. High pressure systems are considerably less transient and of much longer duration than the average low-pressure system – a typical high lasts for 5-10 days (whereas a low-pressure system may be fully spent within as little as 24-48 hours). Sometimes, highs can even remain in the same place for several weeks or more; the weather pattern is then said to be “blocked” (Barriopedro and Garcia-Herrera, 2005). There is currently debate as to whether blocking weather patterns have become more common in recent decades (Barnes et al., 2014).